The invention relates to active acoustic attentuation systems, and provides a system with reduced convergence time to provide faster cancellation of undesired noise.
The invention arose during continuing development efforts relating to the subject matter shown and described in U.S. Pat. Nos. 4,677,676, 4,677,677, 4,736,431, 4,815,139, 4,837,834, and Ser. No. 07/388,014 filed July 31, 1989, all assigned to the assignee of the present invention, and incorporated herein by reference.
Active attenuation involves injecting a cancelling acoustic wave to destructively interfere with and cancel an input acoustic wave. In an active acoustic attenuation system, the output acoustic wave is sensed with an error transducer such as a microphone which supplies an error signal to a control model which in turn supplies a correction signal to a cancelling transducer such as a loud speaker which injects an acoustic wave to destructively interfere with the input acoustic wave and cancel same such that the output acoustic wave or sound at the error microphone is zero or some other desired value. The acoustic system is modeled with an adaptive filter model having a model input from an input transducer such as a microphone, and an error input from the error microphone, and outputting the noted correction signal to the cancelling speaker. The adaptive filter model is characterized by a coefficient weight vector as is known in the art, Bernard Widrow and Samuel D. Stearns, Adaptive Signal Processing, Prentice-Hall, 1985. The error signal from the error microphone approaches a given value, such as zero, as the adaptive filter model responds to the error signal and input signal by updating the weights of the coefficient weight vector from an initial nonconverged value in a nonconverged condition of the filter model at turn-on thereof to a converged value in a converged condition of the filter model. The model continues to adapt to changes in the error signal and input signal.
At turn-on of the model, the vector weights typically start at zero. During the modeling and adaption process, the weights are updated, and eventually change to a final converged value. This final converged value will further change as the input acoustic wave to be cancelled changes.
In various applications, the convergence time of the model is not a significant factor. Typical convergence times may be tens of seconds, or even a minute or two. In applications involving noise from a large industrial fan, a factory, and the like, these convergence times are not objectionable. In other applications, however, faster convergence time may be desirable.
One application where faster convergence time is desirable is the quieting of a motor vehicle exhaust, where substantially instantaneous noise cancellation is preferred.
Another application where faster noise cancellation is desired is in the interior of a motor vehicle where the driver or passengers are exposed to noise introduced by the engine, exhaust system, tires, road, etc., where quick cancellation is desirable.
The present invention is particularly useful in combination with that in noted copending application Ser. No. 07/388,014. The noted copending application involves an intercom communication system with active acoustic attenuation. The noted noise within the vehicle is annoying and prevents enjoyment of the vehicle entertainment system and hinders communication with the vehicle via radio telephone. The present invention in combination with that in the noted copending application cancels the noise and quiets the environment quickly, and also enables enjoyment of the entertainment system and communication to the vehicle via radio telephone, and vice versa.
Another particularly desirable application of the present invention and that in the noted copending application is in magnetic resonance medical imaging systems where the patient is in a noisy environment in the hollow interior tunnel of a cylindrical tubular structure subject to magnetic hum and the like. The patient has difficulty hearing the attendant medical technician, and vice versa, and it is thus difficult to communicate instructions, symptoms, etc. Furthermore, the noise adds to the discomfort of the patient who may already be apprehensive of the medical procedure and the claustrophobic tunnel. The combination of the present invention and that in the noted copending application quickly cancels the noise and quiets the environment within the tunnel, and also enables communication from the patient to the medical technician, and vice versa.
The invention also reduces convergence time of the filter model to new convergence conditions in response to changes of the input acoustic wave. New updated weight values are provided according to a sensed parameter of the acoustic system without waiting for convergence of the filter model to the newly changed input acoustic wave, such that the filter model converges to the newly changed input signal in a shorter time. The invention thus provides reduced convergence time both at initial turn-on of the system and also after the system is up and running and converged and then subject to a new input condition. The latter is particularly useful in systems where the input noise may be subject to rapid and large magnitude changes where the channel gain must be changed and the model must reflect the gain change.